This invention relates to the field of digital to analog converters and, more particularly, to highly accurate, yet simple and inexpensive converters.
In a typical digital/analog transmission system, an analog signal is sampled at a minimum rate of two times the highest frequency in the signal. The samples are quantized and encoded as a pulse code modulated (PCM) signal. The "words" representing the PCM signal are typically 14-16 bits long. The encoded digital signal is then transmitted serially, least significant bit (LSB) first, in the desired fashion. After reception, the analog signal must be retrieved by complementary decoding and a digital to analog (D/A) conversion. The conversion must be done simply, quickly and accurately. Thus, the function of a digital to analog converter is to decode a series of digital bits back into the original analog signal from which the digital bits were derived. Early digital to analog converters utilized a resistive divider or ladder as the heart of the converter. Such networks are shown in many texts such as "Digital Principles and Applications" by Malvino and Leach (McGraw-Hill, Inc. 1969).
One well-known D/A converter which is theoretically satisfactory was designed by Claude Shannon some years ago. In that converter, at each digital bit a capacitor is caused to charge, then discharge exponentially according to an RC time constant, with the value of the charge at the end of a digital "word" representing the original analog value of that word. In practice, however, the basic Shannon converter requires such a high degree of precision in the resistive and capacitive components that it is not practical for use in a consumer product. The Shannon converter and an improvement by A. J. Rack will be described in more detail hereinbelow.
At the present time, there are many integrated circuit D/A converters available on the market but most are relatively expensive to manufacture due to the need for large numbers of internal components and the precision matching of the components.